Double-acting, reciprocating piston pumps are characterized by the capability of pumping material in both directions of the piston stroke. Pumps of this type, such as disclosed, for example, in U.S. Pat. Nos. 3,160,105 and 3,995,966, include a pump body formed with a longitudinally extending passageway which is divided into first and second pumping chambers by a check valve. One of the pumping chambers communicates with a discharge outlet formed in the pump body and the other pumping chamber has an inlet which communicates with a source of the material to be pumped. In one direction of piston movement within the passageway, the check valve is moved to a closed position and material is simultaneously forced out of the first pumping chamber through the discharge outlet in the passageway and scooped from the source of material through the inlet into the second pumping chamber. Movement of the piston in the opposite direction opens the check valve to permit the passage of material from the second pumping chamber into the first pumping chamber where it is ejected through the discharge outlet therein.
Dual-acting, reciprocating piston pumps of the type described above are effective in certain applications, but problems can be experienced in pumping materials having a relatively high viscosity. In order to pump highly viscous materials, the flow passageways through the pump, and particularly through the check valve and between the pumping chambers, must be as clear and unobstructed as possible. If such passageways are relatively small and/or elongated, for example, the pumping of high viscosity materials therethrough tends to create problems of excessive shear, pressure loss between the pumping chambers and "packing out" or the build-up of material along the surfaces of the passageways and/or piston which restricts flow therethrough.
For example, U.S. Pat. No. 3,995,966 discloses a double-acting, reciprocating piston pump in which an elongated piston or plunger is formed with a relatively large diameter, compared to the diameter of the passageway in the pump body between the two pumping chambers, such that a relatively small material flow path is formed between the pumping chambers. This elongated, small flow passageway is unacceptable when pumping high viscosity materials because the problems of excessive shear, pressure drop and packing out mentioned above can be created in this area. Additionally, in the pump disclosed in the U.S. Pat. No. 3,995,966 and others of this type, no means is provided to scrape or wipe material from the wall formed by the passageway in the pump body. While some pump designs include a scraper or the like for wiping material from the piston, the wall of the passageway in the pump body is not cleaned and can accumulate material during operation of the pump.